Excess intake of dietary calories that exceeds energy expenditure leads to an accumulation of white adipose tissue. Obesity ensues when adipose tissue mass exceeds 30 percent of total body weight. Adipose tissue enlargement is accomplished in three ways: 1) adipocyte precursor cells (preadipocytes) replicate, 2) preadipocytes terminally differentiate into adipocytes, and 3) adipocytes enlarge by storing triglyceride. Most of what is known about adipogenesis centers around the mechanisms of preadipocyte differentiation. My interest is in preadipocyte replication, specifically the intracellular signaling mechanisms involved in the mitogenic process. Preadipocytes are unique cells, because of their capacity to replicate and differentiate throughout life in response to the ever changing nutritional milieu. In some situations of excess nutrient intake, preadipocyte replication and differentiation occur out of proportion to adipocyte enlargement. This gives rise to hyperplastic obesity. Hyperplastic obesity is a common feature of massive obesity, develops with early onset obesity, and is resistant to medical therapy. Omental preadipocytes from massively obese subjects replicate rapidly in vitro indicating that there are inherent cellular defects in the mechanism of preadipocyte replication. Our long term goal is to identify these putative intracellular defects. Our short term goal is to investigate the intracellular signaling pathways involved in normal preadipocyte replication. Our preliminary studies in 3T3-L1 cells indicate that basic fibroblast growth factor induces tyrosine phosphorylation of Stat6. Additionally, the focal adhesion protein paxillin is tyrosine phosphorylated in a time course that is nearly identical to Stat6. In a 3T3-L1 culture system, we propose to test the effects of inducers of preadipocyte replication on activation of the Janus kinase-signal transducers and activators of transcription pathway (JAK-STAT pathway) via phosphorylation, nuclear translocation of STATS, STAT-DNA binding to core elements, and preadipocyte replication, in the presence and absence of JAK-STAT inhibitors. Based on the results of our preliminary data, as well as an increasing amount of evidence indicating that the JAK-STAT pathway and focal adhesion proteins are important for cell growth. We propose to investigate the role of the JAK-STAT pathway and the cytoskeletal-adhesion complex in preadipocyte replication.